1. Field of the Invention
The present invention relates to an optical-fiber inspection device and more particularly to an improvement in the extension of its measurable region and an enhancement of its resolution in measuring the distance to a fault point.
2. Description of the Prior Art
There are two well-known optical-fiber inspection device thus far: the Optical Time Domain Reflectometer (OTDR) and Optical Frequency Domain Reflectometer (OFDR). The present invention relates to an optical-fiber inspection device of an OFDR scheme which identifies a fault point in a section of optical fiber by frequency information obtained by detecting the reflected light from the fault point above.
FIG. 1 shows an exaniple of a theoretical configuration drawing of an OFDR using a fiber interferometer in the prior art. A laser beam emitted from laser diode (LD) module 1 is controlled by LD driver 2 to sweep its frequency. The laser beam is equally divided into two by optical coupler 3; one is propagated through a path 3a and incident to an optical detector, photo-diode (PD) 5. The other is incident to a section of optical fiber 4, a device under test (DUT), and reflected at a fault point 4a (e.g., fracture) in the optical fiber. This reflected light is propagated through a path 3b via optical coupler 3 and incident to PD 5. PD 5 detects interference between these two light beams (the interfering means is omitted).
The output of the PD is analyzed with spectrum analyzer 6 employing a method, e.g., Fast Fourier Transform (FFT), and the frequency and intensity of the interference light. In addition, LD driver 2 gives a trigger to spectrum analyzer 6 for the timing of the measurement.
In this configuration, the oscillation frequency of LD 1 is swept linearly so that the sweeping is .DELTA.f per unit time (.DELTA.t). If a time difference tl exists between the light directly incident to PD 5 via the optical coupler and the light incident to PD 5 after being reflected in the DUT, the frequency difference between these two light beams fl is expressed as shown below. EQU f.sub.1 =(.DELTA.f/.DELTA.t).times.t.sub.1
This time difference is proportional to the distance from the light-incident end of the optical fiber to a fault (fault point 4a) where reflection occurs within the DUT and, for example, expressed in the next expression (the lengths of paths 3a and 3b must be the same). EQU t.sub.1 =2.times.L/V
where L is the distance up to a fault point. (As light advances and returns the same distance, 2.times.L is used).
V is the speed of light within the DUT. (Let the speed of light in a vacuum be c, and the refractive index of the DUT be n; then, V=c/n.) PA1 n is the refractive index of the medium PA1 .DELTA.v is the line width of the light source (frequency)
From the above equations, if the sweeping rate (.DELTA.f/.DELTA.t) of the oscillation frequency is kept constant (already known), the distance to the fault point (position of the fault point) in the DUT can be known from the frequency of the interference signal at the PD, and the reflected amount at the fault point can be known from the magnitude of the interference signal. This enables an inspection device for an optical fiber or the like to be configured.
In this case, the measurable distance is limited by the coherence length of the light source used. The coherence length Lc is given by the following equation in a simplified form: EQU Lc=(c/n).times.(1/.DELTA.v)
where c is the speed of light in a vacuum
For example, if an optical fiber where n=1.5 and an LD with a line width of 1 MHz are used, the coherence length Lc is 200 m, that is, up to a distance of 100 m or so can be measured for the one-way distance. To extend the measurable distance, a light source with a narrower line width is required. However, it is not easy to provide a light source whose light emitting frequency can be swept and whose line width is narrow.
As a method to increase the measuring distance, there is a method of extending the path of the reference light. As shown in FIG. 2, it is a method to connect an extended optical fiber 3c with a length h. This makes measurement of a fault point possible in a range from the forward and return length h within a DUT to the coherence length Lc. However, the following problems remain even in such a measuring distance extension.
Although the measuring distance can be extended by implementing measurement replacing the extension optical fiber 3c with longer ones, there are problems that such measurements require a provision of multiple numbers of long optical fibers and has the necessity of incorporating a manual means of replacement or a channel selector or the like for these optical fibers.
For example, to measure a forward distance of 1 km for a coherence length of 200 m, 9 sections of optical fiber with lengths of 200 m, 400 m . . . , 1400 m, 1600 m, and 1800 m respectively become necessary. Although it is possible to obtain a 600 m fiber by connecting two lengths of optical fiber 200 m and 400 m long, a slight reflection at the connection point may cause ghosting.
In addition, in the configuration in FIG. 3 showing another example of an OFDR, the following problem exists: a light beam (e.g., a laser beam) emitted from electro-optic converter 13 (hereafter abbreviated as E/O converter) corresponding to LD1 in FIG. 1 is driven by the output of sweep oscillator 11 given via distributor 12, and controlled so that the intensity-modulation frequency for the output light is swept. The output light is incident to DUT 4, the device under test, via optical coupler 3 and reflected at a fault point (e.g., fracture) in the DUT. The reflected light is incident to an opto-electrical converter 5 (hereafter called O/E converter) corresponding to the PD in FIG. 1, converted to an electrical signal, and input to mixer 14.
The output of the above sweep oscillator 11 is also connected to mixer 14 via distributor 12, and mixer 14 outputs a signal of the frequency difference between the two input signals. This signal is input to frequency analyzer 15 in which the frequency difference is analyzed.
Sweep oscillator 11 is swept linearly so that the frequency change of .DELTA.f per unit time (.DELTA.t) is obtained. The frequency difference between the two inputs of mixer 14 is proportional to the delay time difference. As the delay time difference is proportional to the distance to a fault point in DUT 4, the distance to the fault point can be known by making a frequency analysis and the reflected amount at the fault point, that is, the size of the fault point can be known from the magnitude of the signal.
However, the following problems still exist because a signal based on the reflected light from a fault point located near the light-incident end of the DUT has a lower frequency in the output of mixer 14.
(1) Much noise is contained in the low-frequency components, such as 1/f noise, and, thus, it is difficult to analyze lower-intensity, lower-frequency signals. PA0 (2) It is hard for a spectrum analyzer generally used as a frequency analyzer to analyze low-frequency signals.
In addition, a signal corresponding to the reflected light (the signal to be measured with a frequency analyzer) from a fault point located remote from the end of the DUT to which the incident light is applied may sometimes be difficult to be measured because its frequency becomes too high.
Further, if more than one fault point located close to each other within a DUT exists, and if an E/O converter having a highly coherent output (e.g., a laser beam) is used, there occurs a problem where light beams reflected from fault points located close to each other interfere with each other and generate noise components causing the fact that the fault points cannot be detected with a high resolution.
As seen above, it is difficult for conventional OFDR's to extend the measurable distance and also to perform fault point detection with a high resolution.
The purpose of the present invention is, considering the above discussed points, to realize an optical-fiber inspection device which can easily extend the measurable distance.
Another purpose of the present invention is to make available an optical-fiber inspection device which can detect fault points with a high resolution.